Autosomal recessive polycystic kidney disease (ARPKD) is, among congenital renal disorders; a significant cause of pediatric morbidity and mortality. Affected children suffer from hypertension, progressive renal insufficiency and portal tract fibrosis. The clinical spectrum of ARPKD is widely variable with most cases presenting in infancy. The genetic defect in this disease is unknown and the molecular mechanisms underlying the phenotype remain to be elucidated. The disease appears to be genetically homogeneous, with both the severe perinatal and milder forms mapping to an identical genetic interval on 6p21-12. This proposal is a competitive renewal application that seeks funds to complete the gene's isolation and initial characterization. In the 3 years since the funding was awarded, we have greatly increased the size of our database of ARPKD families and recruited participation of families from genetically homogeneous backgrounds. In parallel, we have saturated the genetic interval with polymorphic markers and used these to refine the position of PKHD1 to an interval of less than 1cM. We have replaced our YAC-based physical map with one comprised of PACs and BACs. The new sequence-ready map has an average STS density of 1/20kb and predicts that the closest flanking genetic markers are approximately 400kb apart. We have also made significant progress in generating a transcription map of the interval. Using a variety of methods, we have identified 19 sets of non-overlapping expressed sequences in the minimal interval, the majority of which appear to be products of novel genes. Studies are presently underway to complete their isolation and characterization. In this renewal, we seek funds to complete the search for PKHD1 and begin characterization of its gene product. In Aim number 1, we propose to refine the genetic map by using new polymorphic markers in our critical recombinants, identifying new recombinants, and analyzing genetically homogenous populations. Aim number 2 proposes to complete the transcription map of the critical interval and use this to guide our selection of candidate genes for mutation analysis. Once PKHD1 is identified, we will characterize the nature of mutations and investigate the relationship between genotype and phenotype in ARPKD (Aim number 3). In parallel, we will produce antibodies to the PKHD1 gene product and characterize the protein's pattern of expression in Aim number 4. Finally, we will create a murine model of PKHD1 using gene-targeting methods.